1. Field of the Invention
The present invention relates to a polygon drawing apparatus and method of drawing a polygon by combining a plurality of triangles, which apparatus and method may be used in an image display system, or the like, for drawing graphics on a memory and displaying the graphics on a monitor.
2. Prior Art
In a conventional method of drawing graphics on a memory, an ideal straight line is expressed by points (displayed grid boxes) on a displayed grid in which vertical and horizontal lines are arranged at equal intervals, and therefore results in a rough, uneven line called jaggy, as shown in FIG. 1. To reduce the jaggedness or roughness, an anti-aliasing method has been proposed, which will be described with reference to FIGS. 2A and 2B.
In this method, where one pixel is considered as one square or displayed grid box in the grid, and a straight line extends over a plurality of pixels, the intensity value of each pixel is determined depending upon the area of the pixel which is covered by the straight line, as shown in FIG. 2A. FIG. 2B shows an example in which the intensity of each pixel is determined on this basis. In this example, a single straight line can be smoothly drawn by expressing the straight line by differences in the intensity.
In the known anti-aliasing method, however, the area of each pixel which is covered by the ideal straight line must be calculated in a complicated manner. Further, in a known method of drawing a triangle, special processing must be performed only on each edge portion of the triangle, which results in reduction in the speed of drawing a triangle.
Also, when three-dimensional graphics or the like are drawn on a memory, a graphical pattern (polygon) is drawn by combining triangles which are the most primitive or fundamental elements. When drawing a quadrangle ABCD as shown in FIG. 3, a triangle ABC and a triangle BCD are separately drawn. In this case, an edge BC common to the two triangles ABC, BCD is drawn twice.
In this type of polygon drawing processing, logical operations (Exclusive OR and so on) and mixing processing are often performed as processing for drawing pixels. In this case, since a portion of the graphical pattern corresponding to the edge BC is processed twice, the intensity values of pixels in the other portions become largely different from those of the pixels on the edge BC, whereby a band or stripe may appear at the portion corresponding to the edge BC.